Chair base



FIG. 1 is a perspective view of the chair base in accordance with my design;

FIG. 2 is a top plan view thereof;

FIG. 3 is an elevational view from the left in FIG. 2; the view from the right is a mirror image;

FIG. 4 is a front elevational view corresponding to FIG. 2; and,

FIG. 5 is an bottom plan view.

The castors shown in the drawings are not part of the design. 

The ornamental Design for a chair base, as shown and described. 